Wide ranges of medical treatments have been developed using implantable devices such as endoluminal prostheses, which are medical devices adapted for temporary or permanent implantation within a body lumen, including naturally occurring and artificially made lumens. Examples of lumens in which endoluminal prostheses may be implanted include arteries such as those located within coronary, mesentery, peripheral, or cerebral vasculature; veins; gastrointestinal tract; biliary tract; urethra; trachea; hepatic shunts; and fallopian tubes. Various types of endoluminal prostheses have also been developed with particular structure to modify the mechanics of the targeted luminal wall.
Stents are one example of an endoluminal prosthesis. Stents are generally cylindrical shaped devices that are radially expandable to hold open a segment of a blood vessel or other anatomical lumen after implantation into the body lumen. Various types of stents are in use, including balloon expandable and self-expanding stents. Balloon expandable stents generally are conveyed to the area to be treated on balloon catheters or other expandable devices. For insertion, the stent is positioned in a compressed configuration along a delivery device. The stent may be fixed to a balloon that is folded or otherwise wrapped about a guide catheter that is part of the delivery device. After the stent is positioned across a lesion, the stent is expanded by the delivery device. For a self-expanding stent, a sheath is retracted allowing expansion of the stent.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion thereby increasing the inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow. Unfortunately, significant proportions of treated vessels re-narrow or collapse soon after the procedure.
To prevent acute vessel narrowing or collapse, short flexible cylinders, or stents, constructed of metal or various polymers are implanted within the vessel to maintain lumen diameter. The stents acts as a scaffold to support the lumen in an open position. Balloon-expandable stents are mounted on a collapsed balloon at a diameter smaller than when the stents are deployed. Stents can also be self-expanding, growing to a final diameter when deployed without mechanical assistance from a balloon or like device.
Stent insertion may cause undesirable reactions such as inflammation, infection, thrombosis, and proliferation of cell growth that occludes the passageway.
To reduce restenosis stents have been developed with coatings to deliver drugs or other therapeutic solutions. Once the stent is positioned in a target site, these coatings offer long-term treatment from the drug by a controlled release of a specific amount of the drug from the surface of the stent. The rate of release depends upon the chemical and or biological composition of the drug and the amount of the drug depends upon the total depth and depth consistency of the drug coating layer on the stent surface. It has been discovered that methods of loading drugs onto implantable devices may be deficient in their current drug-loading and drug-delivery characteristics. In particular, the amount or volume of the drug capable of being delivered to the target site may be insufficient due to the limited surface areas on the stent and the control of the rate of elution is limited by the chemical characteristics of the drug. In addition, during delivery of the stent, any coating exposed to the body lumen can lose a portion of the coating during delivery, either as a result of blood flow over the surface, or by contacting the vessel tissue prior to delivery to the target site. Furthermore, the use of a polymer matrix to control the release of the drug has been shown to lead to inflammatory response and may be linked to the occurrence of late thrombosis within the stent. Reducing or eliminating the amount of polymer required to achieve a desired controlled drug release would therefore be of great benefit.
To increase the amount of the drug that may be deposited on the surface of the stent, the surface of the stent framework has been modified. Such modifications may be the formation of openings in the stent surface to hold more of the drug. For example, reservoirs can be formed into the surface of the stent with the use of lasers or by dimpling the surface.
One problem that has arisen with drug reservoirs within implantable devices is that after implantation the rate of release of the drug from the reservoir is hard to control due to the size of the opening through which the drug is released.
It would be desirable to have an implantable device with drug reservoirs that would overcome these and other disadvantages.